Revisiting the phylogenetic relationships within Sladenia (Sladeniaceae)

Geological and climatic changes associated with the Ailao Shan–Red River shear zone (ARSZ) are thought to have influenced the evolutionary history of many mostly drought‐adapted low‐elevation plant lineages in southwestern China, yet their role in shaping genetic divergence in montane cloud‐forest plants remains poorly understood. Sladenia (Sladeniaceae), a rare tree genus comprising two species distributed across the ARSZ, provides an opportunity to examine the evolutionary consequences of these historical processes. Here, we integrated expanded plastome sampling with newly assembled nuclear ribosomal ITS sequences to reconstruct phylogenetic relationships within the genus. Plastome data recovered an northeast–southwest geographic structure among populations of S. celastrifolia across the Red River, with S. integrifolia being sister to the northeastern lineage. In contrast, the nuclear ITS data supported both species as monophyletic and resolved them as sister lineages, supporting the recognition of S. integrifolia as a distinct species and revealing marked cytonuclear discordance. Ecological niche modeling indicated the contraction of suitable habitat into southern refugia during the Last Glacial Maximum, followed by postglacial expansion. Together, these findings suggest that montane cloud‐forest plants share the east–west phylogeographic structure documented in lowland lineages, while retaining distinct signatures of both tectonic history and Quaternary climatic change. By integrating plastid and nuclear evidence, our study provides new insights into lineage divergence across the ARSZ.